1. Field of the Invention
This invention relates to an apparatus employing a interference, signals in different frequency bands, such as mobile telephone signals and radio broadcasting, and is preferably mounted to a car.
2. Description of the Prior Art
FIG. 1 is a sectional view of a typical conventional car-mount whip antenna 1 in its extended state. This whip antenna 1 is mounted, for example, near the rear trunk of an automobile car body 2, and is used commonly for the transmission and reception of signals for a mobile telephone and the reception of radio broadcasts. An antenna element 3 of this whip antenna comprises a first antenna element part 4 having a round tubular shape, and a second antenna element part 5 formed telescopically disposed within the first antenna element part 4. The antenna element 3 is accommodated in a housing tube 6 fitted in a mounting hole 14 formed in the car body 2. The housing tube 6 is composed of a tubular body 7 made of electric insulating material such as resin, and an outer conductor 8 and an inner conductor 9 made of conductive materials.
The first antenna element part 4 is composed of a sequential connection of a first conductor 15, a phase shifting coil 18, a second conductor 16;, a band separating coil 19, and a third conductor 17. These conductors 15 to 17 and coils 18 and 19 have identical outside diameters. The phase shifting coil 18 functions as a phase shifter on frequency f1 of a mobile telephone, so that the current distribution in reverse phase may be suppressed low, while the normal phase portion is emphasized in the current distribution profile. The band separating coil 19 has a high impedance against frequency f1 of the mobile telephone, and a low impedance against frequency f2 of a radio broadcast.
Therefore, a colinear array antenna is constituted by conductors 15 and 16 an the phase shifting coil 18, which may be used for the transmission and reception of mobile telephone signals. The overall length of the antenna element 3 is used in the reception of radio broadcasts.
A leaf spring 28 is fixed at a lower end part 15a of the antenna element 3. And, by this leaf spring 28 the antenna element 3 is supported so as to be slidable in the axial direction, while it is electrically connected with the inner conductor 9. At an upper end part 6a of the housing tube 6, the outer conductor 8 is fixed to the car body 2 by way of metallic fixing tubes 21 and 22 and fixing plate 23, and hereby connected electrically. The connections of the housing tube 6, fixing tubes 21, 22 and the fixing plate 23 are filled with sealing resin 24, and a nut 25 is screwed thereover.
Beneath the housing tube 6, a connection hole 26 is formed near the lower end part 9a of the inner conductor 9. In the connection hole 26, an inner conductor 12 of a coaxial cable 11 is connected to the inner conductor 9, and an outer conductor 13 of the coaxial cable 11 is connected to the outer conductor 8. The coaxial cable 11 is supported by a cable support member 30 fitted to the outer conductor 8. This coaxial cable 11 is connected to a branching filter 27, and this branching filter 27 is connected to the transmitter/receptor of the mobile telephone and the radio set by the coaxial cable 29a and 29b.
This whip antenna 1 is erected, for example, near the rear trunk of the car body 2. Therefore, there are a large number of restrictions imposed due to the shape of the car body 2, such as, on the width of the rear fender and the size of the mounting hole 14 for mounting the housing tube 6. Besides, if the outer diameter of the antenna element 3 is reduced too much in order to resist the wind pressure while traveling, the tubular body 7 made of electric insulation material becomes thin, and the spacing between the inner conductor 9 and the outer conductor 8 becomes small.
Therefore, as mentioned below, the characteristic impedance Z2 from the upper end part of the housing tube 6 to the lower end part 9a of the inner conductor 9, that is, in the section l2 up to the current feed point P is lowered. On the other hand, if the mobile telephone is used in a state in which the impedance at the current feed point P is mismatched, the signal sent out from the transmitter is reflected, so that the coil in the transmitter may be burnt.
Therefore, by forming the length of this section l2 at about 15 cm or half of the wavelength .lambda.1 of the mobile telephone, the impedance matching is achieved. Therefore, the current feed point P cannot be set at an arbitrary position. Such construction of the whip antenna 1 in accordance with the above-mentioned length restriction causes the following problems.
FIG. 2 is an equivalent circuit diagram in which the whip antenna 1 is used for the reception of frequency modulated (FM) broadcasts. In this antenna element 3, supposing the characteristic impedance of the section l1 projecting from the upper end part 6a of the housing tube 6 to be Z1, and the characteristic impedance of the section l3 of the coaxial cable 11 to be Z3, the characteristic impedance Z1 of section l1 is nearly equal to the characteristic impedance Z3 of section l3, and is, for example, about 50 ohms. Moreover, the characteristic impedance Z2 of the section L2 is expressed as follows, assuming the outside diameter of the inner conductor 9 to be d, the inside diameter of the outer conductor 8 to be D and the specific dielectric constant of the tubular body 7 to be .epsilon.r: ##EQU1##
On the other hand, because of the restrictions by imposed the shape of the car body 2 as mentioned above, there is not a large difference between the outside diameter d of the inner conductor 9 and the inside diameter of the outer conductor 8, and therefore as is clear from eq. (1), the characteristic impedance Z2 in the section l2 is lowered, and the impedance matching between the section l1 or antenna element 3 and the section 13 or the coaxial cable 11 is worsened, whereby transmission loss increases. Accordingly, the length of the section l2 becomes too long to be ignored with respect to the wavelength .lambda.2 of FM broadcast, and the band width is consequently narrowed.
FIG. 3 is an equivalent circuit diagram in which the whip antenna 1 is used for the reception of amplitude-modulated (AM) broadcasts. The length of the antenna element 3 is formed in accordance with the mobile telephone and FM broadcast, so that it is extremely short for the wavelength of AM broadcasts, and the radiation resistance almost becomes null, and the characteristic impedance Z1 becomes capacitative.
Supposing the capacity of section l1 to be C1;, that of section l2 to be C2, and that of section l3 to be C3, the relation between a voltage V1 induced in the antenna element 3 and a voltage V2 at the power receiving end obtained by way of coaxial cable 11 is set forth in the following equation: ##EQU2## where the capacitance C1 of section l1 and the capacity C3 of section l3 are constant, and the power receiving end voltage V2 may be raised by reducing the capacity C2 of section l2. However, the capacity C2 of section l2 is, supposing the specific dielectric constant in a vacuum to be .epsilon..sub.0, expressed as follows ##EQU3## and the ratio of the inside diameter D of the outer conductor 8 to the outside diameter d of the inner conductor 9 cannot be increased too much as stated above, and therefore the power receiving end voltage V2 cannot be increased too much.
FIG. 4 is a sectional view of another conventional whip antenna 31 in an extended state. This long bar-shaped whip antenna 31 is mounted near the rear trunk of an automobile car body 32, and is commonly used for the reception of radio broadcasts and the transmission and reception of mobile telephone signals. An antenna element 33 of this whip antenna 31 is composed in a sequential connection of a first conductor 34, a phase shifting coil 38, a second conductor 35, a band separating coil 39, a third conductor 36, and a fourth conductor 37. The first conductor 34 and the second conductor 35 have a round cylindrical shape; and the third conductor 36 is formed like a cap.
Within a space 43 formed by the first conductor 34, the phase shifting coil 38, the second conductor 35 and the band separating coil 39, the fourth conductor 37 is accommodated. The outside diameters of the first to third conductors 34 to 36, and coils 38 and 39 are identical, and such elements are housed in a housing tube 40 provided in the car body 32.
The housing tube 40 is composed of an electric insulating tube body 40a, an outer conductor 40b, and an inner conductor 40c. An outer conductor 44a of a coaxial cable 44 is connected to the outer conductor 40b, and an inner conductor 454b of the coaxial cable 44 is connected to the inner conductor 40c.
At the high frequency f1 of a mobile telephone or the like, the phase shifting coil 38 functions as a phase shifter, and the normal phase portion is emphasized by suppressing the current distribution in the reverse phase, while the band separating coil 39 has a high impedance, whereby a colinear array antenna is formed by the first conductor 34, the phase shifting coil 38, and the second conductor 35 to be used for the transmission and reception of mobile telephone signals.
At the low frequency f2 of a radio broadcast or the like, the band separating coil 39 has a low impedance, and the first to fourth conductors 34 to 37 and coils 38 and 39 are used as a whip antenna for the reception of the radio broadcast.
Since the portions of coils 38 and 39 exhibit low strength, they are likely to be broken, and they are reinforced by molding resins 41 and 42 thereto. The resin portions 41 and 42 have the same outside diameters as those of first to third conductors 34 to 36 so as not to form an obstruction when the antenna element 33 is put into the housing tube 40.
In the thus composed whip antenna 31, the resin portions 41 and 42 are bulged out, inward in the radial direction of coils 38 and 39, in order to obtain a desired strength. Therefore, such bulging would interfere with the displacement of the fourth conductor 37 into the space 43, and it is difficult to provide resin portions 41, 42 with a thickness sufficient to obtain a desired strength. Besides, after the coils 38 and 39 are once molded with resins 41 and 42, it is difficult to adjust the length of the coils 38 and 39. Furthermore, since the first to third conductors 34 to 36 are metallic, thus being of material different from the resin 41 and 42, the antenna is deemed to be unaesthetic.
FIG. 5 is a block diagram of a conventional transmission/reception apparatus 50 for a mobile telephone. For mounting a mobile telephone on an automobile, the antenna provided for the reception of radio broadcasts is shared because its transmission frequency band f1 is different from the frequency band f2 of the radio broadcasts. In order to share the antenna in this way, the signal line of the mobile telephone is connected wit the signal line of the radio set. Therefore, when a radio broadcast is received while using the mobile telephone, the so-called beat noise is mixed in the sound reproduced by the radio set. To prevent the generation of such beat noise, the elements shown in FIG. 5 have been used hitherto.
The frequency band f2 of radio broadcasts is, in AM broadcasts, frequency band f2a, that is, 500 to 1620 kHz, and, in FM broadcasts, frequency band f2b, that is, 76 to 90 MHz. In the mobile telephone, on the other hand, for radio communication with the ground station connected with the telephone line, a frequency band fl1a of 870 to 9890 MHz is used in receiving, and a frequency band f1b of 920 to 940 MHz is used in sending. The prior art shown in FIG. 5 makes use of such a difference in frequency band.
In other words, a radio set 51 is connected to an antenna 53 by way of a low pass filter 52, and the mobile telephone 54 is connected to the antenna 53 by way of a high pass filter 55. The signal line connected to the mobile telephone 54 is joined to the signal line connected to the radio set 51. During use of the mobile telephone 54, since the frequency band f1 of the signals transmitted or received by the mobile telephone 54 is relatively high, the radio set 51 will not generate beat noise by the interference with the signal in the frequency band f2 used in the mobile telephone 54 owing to the low pass filter 52.
The equivalent circuit of the antenna 53 and the typical circuit composition of the low pass filter 52 are shown in FIG. 6. A capacitor C11 is connected in series to a signal source 56, and coils L11 and L12 are connected in series to this capacitor C11. The contact point 57 of coils L11 and L12 is grounded by way of another capacitor C12.
The relation between voltage V11 generated in signal source 56 and output voltage V12 of the low pass filter 52 due to electrostatic capacity of capacitors C11 and C12 is as follows: ##EQU4## That is, in the low pass filter 52, since the capacitor C12 is provided between the signal line and the ground, the output voltage V12 of the low pass filter 52 unfavorably becomes smaller than the generated voltage V11 in the signal source 56. In eq. 4, since it is supposed that radio broadcasts are to be received, the attenuation of signals by coils L11, L12 is assumed to be sufficiently small.
FIG. 7 is an equivalent circuit diagram in the frequency band f2a of AM broadcast of an antenna 61 and a cable 62 in a different prior art device.
In a car-mounted radio set, it will be very convenient if FM radio signals, AM radio signals, and mobile telephone signals can be received by one antenna. In an antenna which is extended or retracted by a motor or the like, a signal cable cannot be attached to the lower end of the antenna, and it is difficult to shorten the signal cable. Accordingly, the cable capacity of the signal cable increases, and the impedance derived from the cable capacity becomes high. In particular, in radio signals of a relatively low frequency band such as AM radio signals, the effect of cable capacity becomes larger. Therefore, in a car-mounted antenna, signals in a wide frequency band must be sent out to the radio set while suppressing the loss by the signal cable.
The antenna 61 can be represented by antenna effective capacity Ce and antenna reactive capacity Ca, and the AM radio signals received by this antenna 61 can be represented by an alternating-current power-source V21. The cable 62 can be shown as a line l11 between terminals A1 and B1, and this line l11 is grounded by way of cable capacity Cb. The signal at the terminal B1 is fed into a radio set. The voltage V22 at this terminal B1 is expressed as follows: ##EQU5## As expressed in eq. 5, supposing that the cable capacity Cb is large, the gain of the AM radio signals of relatively low frequency received by the antenna 61 is lowered so that the cable capacity Cb makes the receiving sensitivity and the ratio of signal to noise (S/N ratio) drop.
To prevent such a drop in receiving sensitivity and S/N ratio, an amplifier is placed between the antenna 61 and the cable 62 that is, at the position of terminal A1, so that the receiving sensitivity and S/N ratio are improved. In such antenna, since active elements are used, they give rise to an increase in cost, and also involve other problems such as maintaining a circuit characteristic of suppressing only the distortion of signals at the time of input of a strong electric field. In addition, new problems may be also experienced, such as loss due to impedance conversion in the amplifier, and insufficient matching of impedance.